Introduction
This article will be useful if you need to consume ASP.NET webservices from languages or platforms without .NET support. In this example, I am consuming a webservice from a Java applet, but it would take very little work to adapt my code to run as a Java application. Although I have only tested this on Windows XP, the code should run on other platforms, providing cross-platform client-side consumption of ASP.NET webservices.
I developed this solution because I needed to add complex client-side processing to my ASP.NET application. I did not want to have to cope with all of the slightly different JavaScript implementations used by the main browsers, and this led me to consider a Java applet.
This article shows you how to use a Java applet in your webpage and get it to communicate back to ASP.NET by consuming an XML webservice on the client.
Background
There are, no doubt, many reasons why somebody might want cross-platform consumption of ASP.NET webservices, but this is my story:
I have an ASP.NET application where I needed to add client side image manipulation facilities. I looked into writing the client-side code in JavaScript which seemed like the obvious choice, but was frustrated by different implementations in the browsers, especially when trying to use image manipulation features. For example, if you want to alpha blend your image, you would have to use img.style.MozOpacity in Netscape, img.filters.alpha.opacity in Internet Explorer and there is no way to do this at all in Opera. I really didn't want to have to write special code for each browser or downgrade the functionality to the lowest common denominator.
Therefore, I decided to try writing my client-side code in Java 1.1. Note that I'm using the Java 1.1 SDK, as Internet Explorer currently does not support code developed with Java SDKs after 1.1.4 (although it seems to me that Java SDK 1.1.7 code seems to work OK in IE). A Java applet can do all of the image manipulation I needed and is supported by many browsers. Finally, I don't have to write separate code for each browser.
The next problem was, how do I get the result of the data processing done in the Java applet back into my ASP.NET application? The answer that I came up with was to create an ASP.NET webservice and consume it in the Java applet.
A Simple Example
Take a look at the following very basic webservice:
[WebMethod]
public string ConcatWithSpace(string one, string two)
{
return one+" "+two;
}
It could be consumed very simply by making the Java applet perform a request to: webservice.asmx/ConcatWithSpace?one=FirstValue&two=SecondValue... But what if we wanted to send larger amounts of data to the webservice? How would we go about implementing a SOAP request and response in a Java applet...? There are probably many options if you are using a more recent Java SDK, but because I want to support many browsers including Internet Explorer, I'm stuck with the Java 1.1 SDK. However, this gives us a better opportunity to understand how to talk to ASP.NET web services on other languages and platforms. Therefore, I have written my own SoapRequestBuilder class in Java that makes it quite easy to consume a service and return a single string response. Take a look at this example, which consumes the webservice above:
SoapRequestBuilder s = new SoapRequestBuilder();
s.Server = "127.0.0.1";
s.MethodName = "ConcatWithSpace";
s.XmlNamespace = "http://tempuri.org/";
s.WebServicePath = "/SimpleService/Service1.asmx";
s.SoapAction = s.XmlNamespace+s.MethodName;
s.AddParameter("one", "David");
s.AddParameter("two", "Hobbs");
String response = s.sendRequest();
In this example, the response string is filled with the result from the webservice after we sent it two parameters. The full source code of the SoapRequestBuilder class is included in the source files.
That's it! It's only a very simple example and at the moment, I'm only interested in returning a single value back from the webservice, but I have used something similar with a webservice that talks to my back-end database and this allows my Java applet to store data which can then be accessed by the ASP.NET pages.
To run the example code, set up the webservice first, then copy the Java applet files to any folder on the same machine and double-click the HTML file. All of the parameters are configured inside the HTML file, so you can change the values passed to the webservice by editing the HTML; you should not have to re-compile the Java source. I have tested the example in IE 6.0, Netscape 7.1 and Opera 7.23 on Windows XP.
Notes
Some Java Virtual Machines (like the Microsoft one) only allow you to make a socket connection to the same machine that hosts the Java class files. Therefore, if you're using an applet like me, you will need to host the Java class files on the same machine where the webservice resides.
